The progress in information society draws attention to optical communication technique as a technique that realize high-speed communication. Meanwhile, the optical communication technique has been studied to be improved to attain a faster-speed communication.
A super-high speed optical switch, which switches at a faster speed than conventional optical switches, is inevitable for attaining the faster-speed communication in the optical communication technique. In order to realize such a super-high speed optical switch, it is important to develop a material having a large optical non-linearity and a faster response speed.
Here, the non-linearity is a phenomenon that light irradiation changes absorption coefficient and refractive index of a material on which the light is irradiated. Thus, irradiation of light on a material having optical non-linearity can prevent another light from passing through the material, or change a direction of the another light. That is, the use of the material having optical non-linearity makes it easier to construct an all-opt switch, which controls light by using light.
However, conventionally-known materials having large nonlinear optical responses (optical non-linearity) have slow responding speeds. This hinders practical use of the all-opt switch. That is, a material having a large optical non-linearity and a fast responding speed is necessary to realize an all-opt switch.
Group I-VII semiconductors such as CuCl and the like have drawn attention as such a material having a large optical non-linearity and a fast responding speed. Group I-VII semiconductors (such as CuCl and the like) has a very small Bohr radius of exciton (approximately 0.7 nm), and a very large binding energy of exciton (approximately 200 meV). The very small Bohr radius of exciton and very large binding energy of exciton allow stable exciton state. Because of this, it is expected that optical non-linearity of group I-VII semiconductors becomes higher due to electric field or confinement of excitons. Therefore, optical properties of the group I-VII semiconductors have been extensively studied (see S. Yano, T. Goto, and T. Itoh, J. Appl. Phys. 79(1996) p. 8216).
For example, A. Ekimov, Al. L. Efros, and A. A. Onushchenko, Solid State Commun. 56 (1985) p. 921, and T. Itoh, Y. Iwabuchi and M. Kataoka, Phy. Stat. Sol. B 145 (1988) p. 567, CuCl studied optical properties of a structure in which single crystal particles are embedded in a glass matrix or NaCl matrix. G. R. Olbright and N. Peyghambarian, Solid State Common. 58 (1986) p. 337, R. S. Williams, D. K. Shuh and Y. Segawa, J. Vac. Sci. Technol. A6 (1988) p. 337, A. Kahn, S. Ahsan, W. Chen and M. Damas, Phys. Rev. Lett. 68 (1992) p. 3200, and A. Yanase, Y. Segawa, Surf. Sci. 278 (1992) L105 examined surface morphology and optical properties of CuCl epitaxial growth on various substrates such as NaCl (001) substrate, CaF2 (111) substrate, GaP (110) substrate, MgO (001) substrate, and the like.
Furthermore, recently it is theoretically predicted (see H. Ishihara, T. Amakata, K. Cho, Phys. Rev. B65, 2001, 035305) that a CuCl thin film (CuCl nano structure) having a nano-level planarity has a large optical linearity due to an internal electric field that is resonance-increased when the CuCl thin film has a particular size (film thickness of 26 nm). The prediction expects that wavelength of light coupled with electrons would be very short in the crystals and be interfered with even the film thickness of nano scales, and the interference would result in the increase in the internal electric field when the film has a film thickness that is near a particular film thickness. The particular film thickness that causes the resonance-increase of the internal electric field is unique to each material. It is expected that the value of the particular film thickness of a material be determined depending on contribution of the electronic resonance order in the material or dielectric constant of the material.
In view of this, there is a demand for a technique for producing, from a group I-VII semiconductor, a single crystal thin film that is excellent in planarity and crystallinity.
However, it is difficult to produce a single crystal thin film having a nano-level planarity (nano structure) from a group I-VII semiconductor such as CuCl or the like. That is, a technique has not been realized conventionally, which control a size, shape, planarity, and exciton attenuation constant in a thin film made from a group I-VII semiconductor such as CuCl or the like.
For example, a molecular beam epitaxy (MBE) method has been established as a method for producing a nano structure from a group III-V semiconductor, but not for a group II-VI semiconductor and a group I-VII semiconductor.
Group II-VI semiconductors and group I-VII semiconductors are more ionic than the group III-V semiconductors, thereby making it difficult to use the MBE method for the group II-VI semiconductors and group I-VII semiconductors. That is, in order to produce, on an ionic single crystal substrate, a flat single crystal thin film (flat single crystal super thin film) in a nano thickness from a group I-VII semiconductor, which is an ionic semiconductor, it is necessary to establish a technique different from the III-V semiconductor crystal growth which is applied for III-V semiconductor having a strong covalent bond.